'03 E350 SMB with Dana ProRock front diff. When turning a corner at 5 to 10 mph I can hear a distinct solid metal to metal clunk. The sound lasts just a second and can be heard while sitting in the drivers seat. There is no significant, corresponding vibration of the steering wheel. The sound does not happen every time I turn and can be triggered by braking hard at speeds below 30mph.

The van steers and tracks well. No crashes or running over large objects in the past 7 months. Calipers are bolted on, wheels are torqued to spec, shocks are not leaking and bolted in place, no play in the hub assembly (6 years ago I had an outer bearing disintegrate) and all of the suspension and steering components appear to be OK - no endplay or looseness.

I do not know what a failing rack and pinion would sound like but the steering fluid is good. Need more information? Any idea where to look?

I don't think any of the E-series vans have rack and pinion steering, so it's probably not that.

I would check track bar, ball joints, and radius arm bushings. If you're not familiar with all of those, it might be easiest to take it to a shop that specializes in front end repairs and rebuilds. I bet they can figure it out pretty quick.

My front end uses leaf springs, and when turning, the leaf springs "fan", that's to say they slide across one another a bit, and that causes some loud "clunk's" as they tend to stick then release. It's been going on for years though and besides the noise it's not a problem. Just a thought...................

I did not consider the leaf springs as a source for the odd sound. I will look at them more closely. I know that the shackles are good. The original track bar broke 4 years ago. It sounded like one of the long tubes on a door chime when it was rattling around. Bushings seem ok. Had the ball joints checked 6 months ago. Looked OK but 1500 miles might have made a difference. Ball joints are a real pain in that I do not get more than 50K out of them. Looks like I've gotten over 60K with this set. NAPA I think. Will jack it up to make sure. I cannot see any unusual movement in the tie rod or pitman end pieces. All of the fittings have been greased. I know that because the guys do not wipe off the excess. The mess is reassuring.

If you are going through ball joints that often you might try replacements from Dynatrac.
Are you sure the noise is coming from the front. We had a similar sounding noise for years and only at low speeds. We live an a semi-remote area but found a great mechanic. The problem turned out to be loose clutch discs in the rear diff. If you can reproduce the noise at slow enough speeds get someone to walk along outside while the noise is active. It was very hard for us to determine where the noise was coming from while driving. Turned out the discs had come loose (broken) and were rubbing against the differential case. Basically the entire rear diff needed a rebuild including the case. At the recommendation of our mechanic we had him get a Jasper rebuild Dana 60. No more clucking and a smaller bank account..